Relief Scenarios
Todd Jekel, Ph.D., P.E.
Industrial Refrigeration Consortium
Research & Technology Forum
May 8-9, 2013
Madison, WI
5 6 7
In this presentation, I will
discuss
The purpose of scenarios in relief
system vent piping design
Common scenarios to consider
Complications
Examples
Scenarios
Defined
− conditions enabling one or more relief
valves on a common header to
simultaneously relieve pressure
Scenario categories include
Maintenance
Relief
Scenarios
Operational Fire Condition
Upset
ASHRAE 15 “Scenarios”
Basis for sizing the relief valves
− Vessels (§9.7.5) isolated & subject to a fire
No insulation or jacket assumed
− Compressors (§9.8 & Appendix F) “cold-
start” with the discharge isolation valve
closed
Minimum regulated flow with suction
assumed to be +50°F for high-stage duty and
the saturated intermediate temperature for
boosters
§9.7.8.4 discusses common headers and
relief valves “expected to operate
simultaneously”
IIAR 2
11.3.4 ... The size and maximum
equivalent length of common
discharge piping downstream from
each of two or more relief devices shall
be governed by the sum of the
discharge capacities of all the relief
devices that are expected to
discharge simultaneously…
Relief header scenarios
The extremes, often unrealistic
− Only ONE PRV relieving at a time
− EVERY PRV simultaneously relieving
EVERY PRV lifting is not a SCENARIO
but it does provide a RESULT
What is the SCENARIO?
− Fire in the area surrounding the
components
− Maintenance related scenario
− Internal heat addition
− Other?
The Extremes
Only “ONE PRV relieving at a time”
− Issues
ASHRAE 15 “scenarios” are for sizing the
relief valve (isolation)
ASHRAE 15/IIAR 2 refers to multiple relief
valves relieving simultaneously
− Observations
Every PRV needs to be present in at least
one scenario
Only one PRV relieving at a time is often
not reasonable for external heat addition
scenarios (i.e. fire)
The Extremes
“All at once” simultaneous relief scenario
− Issues
Doesn’t provide any additional insight into
actual scenarios
May (probably) not be a realistic scenario
Causes us to ignore possible lessons
learned by thinking through more realistic
scenarios
− Observations
Provides for an apples-to-apples
competitive bid
No one can say you haven’t considered
every possible overpressure scenario
Scenario identification
Use the Process Hazard Analysis (PHA) to
identify/corroborate scenarios
− Overpressure of system components or
plant areas (e.g. machinery room)
Maintenance procedures should be
reviewed to identify potential overpressure
scenarios
Previous incidents that have occurred in
the facility or the industry at-large should
be used to identify overpressure scenarios
Scenario complications
Component on common pressure level with
different MAWPs¹
− Examples
Compressor oil separators (or oil coolers) with
300 psig MAWP discharging into an vessel
with <300 psig MAWP
Booster compressors w/higher MAWP
discharging into an intercooler w/lower MAWP
Sizing basis for internal heat sources
− Examples
Thermosiphon oil coolers
− Recommend sizing relief valve for full-load oil
cooling heat load
Components undergoing Clean-In-Place (CIP)
¹ Maximum Allowable Working Pressure
Machinery room scenarios
Fire
− What components are nearby?
− What pressure levels are they on?
− What are the component MAWPs?
Maintenance
− Compressors are the most common
maintenance scenario
− ASHRAE sizing scenario
How many compressors would
realistically be undergoing this scenario
simultaneously?
Example: Machinery Room
Scenario #1: Machinery room fire
− All vessel relief valves in the room active
− Heat gain to components with higher
MAWP on the same pressure vessel is
considered
Thermosiphon oil coolers heat gain (fDL)
added to pilot receiver relief valve
Oil separators heat gain (fDL) added to
high pressure receiver or intercooler
Example: Machinery Room
Scenario #2-#n: Compressor maintenance
− Each compressor’s relief valve active for the
largest number of compressors expected to
undergo simultaneous maintenance
− Include the oil cooler if the relief valves
fitted on the oil cooler and oil separator
share a common riser to the header
− Note that each compressor may have it’s
own scenario if the branch piping details and
relief valve sizes are different
Other non-simultaneous
justifications
Fire-rated partition separating
components on a common header
− For example, a high-pressure receiver
located outside of machinery room
Components are normally in common
(i.e. not isolated relative to one another) with
different MAWPs
− For example, a 300 psig oil pot that is
connected to a 250 psig recirculator
package
Mitigation example
Take steps to reduce the scenario or
the size of the required relief valve
Background
− CIP of refrigerated silo
− ASME Stamped at 150 psig MAWP
− CIP has lifted relief valves in the past
even when steps are skipped and heat
exchanger is open to suction
− Unknown how much the pressure
exceeded the relief valve set pressure
Mit. Example, continued
An incident investigation should have been
done in response to lifting relief valve
− Possible investigation recommendations
Revisit/re-do PHA on silo
Train staff responsible for CIP
Install a high-pressure cutout to stop CIP if
pressure reaches a pre-determined limit % of
MAWP
Verify CIP process, controls, and equipment
are being maintained as part of mechanical
integrity
Automate the CIP process
− Wrong response
Increase the size of the relief valve
Recommendations
If you use an “all at once” scenario for
designing the header, consider
describing and highlighting it as such
and creating reasonable scenarios for
showing compliance
Use the PHA process to identify
scenarios
EVERY relief valve on a header MUST
BE part of AT LEAST ONE scenario
− Without calculating the back pressure
you cannot show compliance
For more information,
See two part series in IRC Cold Front:
Vol. 12 No. 1 and Vol. 12 No. 2
